add appropriate attributes to Utf8TextContentBlock

Author: Tyler-IN

src/ModelContextProtocol.Core/Protocol/ContentBlock.cs

@@ -479,14 +479,15 @@ public string Text
 public sealed class Utf8TextContentBlock : ContentBlock
 {
     /// <inheritdoc/>
+    [JsonPropertyName("type")]
     public override string Type => "text";
 
     /// <summary>Gets or sets the UTF-8 encoded text content.</summary>
     [JsonIgnore]
     public required ReadOnlyMemory<byte> Utf8Text { get; set; }
 
     /// <summary>Gets the UTF-16 string representation of <see cref="Utf8Text"/>.</summary>
-    [JsonIgnore]
+    [JsonPropertyName("text")]
     public string Text
     {
         get